Tibial Component Overhang Greater than 2 mm Should Be Avoided in Unicompartmental Knee Replacements: An In Vitro Robotic Study

INTRODUCTION: Tibial implant overhang has been identified as a possible problem in unicompartmental knee replacements (UKR) and to some degree in total knee replacements (TKR). The idea of an optimally placed tibial implant is that the implant would be well supported by the cortical rim of the proximal tibia without overhanging. Overhang on the medial side can cause irritation of soft tissues and medial collateral ligament (MCL) impingement which may lead to chronic pain and possibly even damage the MCL. The surgeon must determine the most appropriate size of the component without the risk of overhanging or subsidence. Knowledge is very limited as to the amount of tibial component overhang and its resulting effects on the MCL (1). To our knowledge there are no in-vitro studies investigating the association of tibial component overhang and MCL impingement. We conducted an in-vitro robotic study to quantify the load changes in the human MCL during passive flexion-extension (PFE) with different amounts of overhang and to define a safe overhang limit. We also investigated the relationship between flexion angle and load changes in the MCL. We hypothesize that there will be a statistically significant effect of tibial component overhang on the MCL load. It is also hypothesized that load changes in MCL at different flexion angles will be significant.